This invention relates generally to vehicle suspension systems, and more particularly to hybrid leaf springs incorporating reinforced bond lines between leaf spring layers.
Known leaf springs are constructed from several elongated strips or leaves of metal stacked one-on-top-of-the-other in a substantially parallel relationship and then clamped together. Typically, these springs are employed in vehicle suspension systems in one of two different load carrying configurations, cantilevered, or three-point-bendingxe2x80x94the latter being the more common method of use. A cantilevered leaf spring is one where the leaf spring is fixed or supported at one end to the frame of a vehicle and coupled to an axle at its other end. Alternatively, a leaf spring mounted in three-point-bending is supported or fixed at one end to a structure with the other end mounted such that it can float, and the load is supported by the spring between its two ends. The use of leaf springs mounted in three point bending is so widespread that the Society of Automotive Engineers (SAE) has developed a formal leaf spring design and use procedure.
Metal leaf springs constructed in the manner described above are incorporated into a variety of different vehicle suspensions including, automobiles, light to heavy trucks, trailers, construction equipment, locomotives, and railroad cars. They are also employed in recreational vehicles, such as bicycles, snowmobiles, and ATV""s (all terrain vehicles). The leaf springs mounted on the vehicles listed above, function to improve the smoothness of the vehicle""s ride and to absorb and store energy for release in response to bending and/or impact loads imposed on the spring resulting from such things as encountering obstructions in a road during the vehicle""s operation.
The mechanical properties defining a vehicle suspension system, particularly the spring rate and static deflection of the leaf springs, directly influence the smoothness of the vehicle""s ride. Generally, a smooth ride requires the leaf springs to have large static deflections. The smoothness of the ride is also influenced by the vibration damping characteristics of the leaf springs. Damping is a parameter that quantifies the ability of the leaf spring to dissipate vibratory energy. Therefore, a high degree of damping is desirable in leaf springs used in automobiles to minimize the vibratory amplitudes transferred to the passenger area.
The ability to accurately determine the mechanical properties and performance characteristics of a leaf spring is critical to the proper design of vehicle suspension systems. One of the problems resulting from the construction of conventional leaf springs is that the variable lengths of the stack of individual leaves creates a stepped spring construction that only approximates constant stress, the steps tend to create localized areas of high stress known as stress concentrations which detrimentally affect the load carrying capability and useful life of the leaf spring. In addition, the fact that the springs are composed of lengths of metal stacked one-on-top-of-the-other causes them to be quite heavy, this additional weight causes a concomitant reduction in fuel economy.
Moreover, because it is impossible to predict the exact conditions and stresses that a leaf spring will be subjected to, the fatigue life of the spring is generally limited. This problem is further exacerbated by the build-up of foreign material on and between the individual leaves. Not only does this cause corrosion, thereby weakening the leaf spring and making it more susceptible to fatigue failure, but it also affects the stiffness of the leaf spring and hence the smoothness of the ride of the vehicle in which the spring is employed. Generally the magnitude of the contribution made to the strength of a particular leaf spring due to inter-leaf friction is determined empirically. When foreign material gets between the leaves it can dramatically increase, in the case of particulate matter, or decrease, in the case of oil, the friction between the leaves, thereby altering the original mechanical properties of the spring. In addition, the shear conductivity between the leaves, which is a measure of the amount of shear stress transferred from leaf-to-leaf, is typically low in conventional leaf springs because the individual leaves are only clamped at the ends. Therefore, the stress transfer capability along the length of the spring is dependent on the aforementioned inter-leaf friction.
In many applications, leaf springs are loaded not only by vertical forces but also by horizontal forces and torques in the longitudinal vertical and transverse vertical planes. These forces are typically generated when the brakes on the vehicle incorporating the leaf spring are applied. The aforementioned horizontal forces and torques cause the leaf spring to assume an xe2x80x9cSxe2x80x9d shaped configuration, a phenomena referred to as xe2x80x9cS-ingxe2x80x9d. The stresses induced in the spring when this occurs can be quite high. In order to minimize S-ing in a leaf spring, the stiffness of the spring must be increased, however, this can detrimentally affect the smoothness of a vehicle""s ride.
In order to address the above-described problems, those skilled in the art have attempted to fabricate purely composite leaf springs, wherein the individual leaves are formed from a composite material of the type consisting of a plurality of fibers embedded in a polymeric matrix. However, while these springs offered significant reductions in weight, as well as increased fatigue life and damping, their cost was prohibitive. The composite springs were also difficult to attach to the frame of a vehicle and required the use of special adapters.
This inventor developed a hybrid leaf spring to answer the above-mentioned drawbacks as set forth in U.S. Pat. No. 6,012,709, the disclosure of which is herein incorporated by reference. The hybrid leaf spring includes an elongated primary leaf element having a first modulus of elasticity. One or more layers of composite material is bonded to the primary leaf element in order to provide a light weight, durable and cost effective leaf spring having anti-S-ing capability and increased shear conductivity.
However, as a result of more stringent product fatigue durability requirements, there is a need to improve the bond strength between layers of the hybrid leaf spring.
Based on the foregoing, it is a general object of the present invention to provide a leaf spring that overcomes the difficulties and drawbacks of prior art leaf springs.
It is a more specific object of the present invention to provide a hybrid leaf spring with improved bond strength and fatigue properties.
In a first aspect of the present invention, a hybrid leaf spring includes an elongated primary leaf having a compression surface, an opposite tension surface, and a first modulus of elasticity. At least one composite material layer is provided having a second modulus of elasticity different from the first modulus of elasticity. An adhesive layer is interposed between and bonds the at least one composite material layer to and in substantially parallel relationship with a respective one of the tension and compression surfaces of the elongated primary leaf. A reinforcing layer of sheet material extends within the adhesive layer, preferably in spaced relation to opposing surfaces of the primary leaf and the composite material layer to strengthen the bond formed by the adhesive layer.
In a second aspect of the present invention, a hybrid leaf spring includes an elongated main spring component, and a second stage or overload spring component coupled thereto. The second stage or overload spring component includes a composite material layer, a metallic layer having a thickness less than that of the composite material layer, and an adhesive layer interposed between and bonding the composite material layer to the metallic layer. A layer of reinforcing sheet material extends within the adhesive layer, preferably in spaced relation to opposing surfaces of the composite material layer and the metallic layer of the second stage or overload spring component to strengthen the bond formed by the adhesive layer.